These sensors are used for example in systems for observation of the Earth by satellite. They comprise several parallel rows of photosensitive pixels; the sequencing of the control circuits for the various rows (exposure time control then reading of the photogenerated charges) is synchronized with respect to the relative advancement of the scene and the sensor, in such a manner that all the rows of the sensor see a single line of the scene being observed. The signals generated are subsequently added together point by point for each dot of the line being observed.
The theoretical signal/noise ratio is improved by the ratio of the square root of the number N of rows of the sensor. This number can go from a few rows to a hundred or so rows depending on the application (industrial testing, Earth observation, panoramic dental X-rays or mammography).
In addition, the non-uniformities in sensitivity of the pixels of the same row bar, and the non-uniformities in dark current of the pixels, are reduced as a consequence of the averaging which results from the addition of the signals from the various rows.
In the image sensors using charge transfer (charge-coupled device or CCD sensors), the addition of the signals point by point took place naturally and without read noise by emptying into one row of pixels the charges generated and accumulated in the preceding row of pixels, in synchronism with the relative displacement of the scene and the sensor. The last row of pixels, having accumulated N times the charges generated by the image line being observed, can be read.
The standard technology of CCD image sensors uses high power supply voltages and consumes a large amount of power; this technology is based on the use of adjacent and mutually overlapping gates of polycrystalline silicon.
The technology of image sensors has subsequently evolved toward sensors with active pixels using transistors, which will hereinafter be referred to as CMOS sensors for simplicity because they are generally fabricated using CMOS (complementary-metal-oxide-semiconductor) technology; in these CMOS sensors, there is no longer any transfer of charges from row to row toward a read circuit or a register but there are active pixels with transistors that collect photogenerated electrical charges and convert them directly into a voltage or a current. The various rows of the sensor therefore successively supply voltages or currents representing the illumination received by the row. These structures do not allow summations of these currents or voltages to be performed without noise; it is therefore difficult to produce a time-delay charge integration linear sensor. The fabrication technology is however simple, it has low power consumption and it operates at low voltage.
Attempts have however been made to fabricate CMOS time-delay charge integration linear sensors.
In particular, the use of switched capacitors has been tried in which successively received currents are integrated, thus accumulating charges received from several pixels in a column onto the same capacitor (U.S. Pat. No. 6,906,749, WO0126382).
Another solution provided is to convert the signals coming from a row of pixels into digital values, to sum the digital value corresponding to the pixel of rank j of the row in an accumulator register of rank j which accumulates the digital values corresponding to the pixels of same rank j from N successive rows (patent FR2906080).
Solutions using an accumulation of charges inside of the pixel have also been provided, for example in the patent publication US2008/0217661. They use a technology more complex than that which is strictly necessary for fabricating image sensors in CMOS technology, or else they suffer from losses during the transfers of charges.
In the patent publication FR2960341, a sensor is provided using CMOS technology with a single gate level of polycrystalline silicon, using an alternation of gates and photodiodes. The structure relies on an asymmetry of the gates in order to impose a direction of transfer common to all the charges, so as to avoid the charges going off randomly in one direction or the reverse direction. Owing to this intentional asymmetry, it is excluded to be able to choose a direction for transfer of the charges in the opposite direction to the direction imposed by the asymmetry. However, in some applications, the user would like to be able to reverse the direction of accumulation of the charges. This is the case, for example, in a scanner operating in TDI mode and that needs to be able to work with both opposing directions of scanning without reversing the orientation of the sensor with respect to the image.